Methods of improving extrusion properties of lead-antimony alloys



E. J. LARSEN 0F LEAD-ANTIMONY ALLOYS Filed July 19, 1961 mm oI. N l

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Dec. 3, 1963 METHODS oF IMPRovING ExTRusIoN PROPERTIES INVENTOR.

E. J LARSE N kan? QQ u NmN @SQ @fbx QOW A 7' TORNE V When the melt is cooled down to temperatures corresponding to a point f3, which is the point of intersection of the line x-x with the Solidus line and corresponds to temperatures of `about 580 F., the liquid phase is no longer possible for this composition and all of the original melt will become solid. The last drop of alloy to freeze at this temperature will have composition corresponding to a point te on the Liquidus line which corresponds to about 3.3 percent of antimony. Any further cooling of the now solid alloy down to about 45 0 l?. results in cooling of the solidified alloy to the desired extrusion temperature. The point 13 gives also the average composition of the melt which has solidified, which is one percent of antimony by weight of alloy.

The above freezing has been described ias an empirical freezing which takes place when an alloy being cooled is at rest. A grain or a crystal starts its growth with a normally dendritic structure, the framework of which is composed of the purer material surrounded by the liquid which is richer in the alloying material, in this case antimony, so that a mushy or a spongy mass is formed filled with liquid which continues to freeze until the whole mass is frozen solid.

When the same material is agitated during cooling, for example, when the alloy is cooled Iwithin `the extruding apparatus while being advanced by the screw impeller, the spongy material is kneaded whereby the liquid is separated from the solid part. During the extrusion, the frozen alloy portion is picked up by the screw impeller of the extrusion apparatus ll and the liquid alloy portion rich in antimony is left behind, resulting in the separation of the original alloy into alloys of various instantaneous cornpositions similar ito a-zone refining separation process. The instantaneous remaining antimony-rich liquid alloy portion of the original lead-antimony alloy then is cooled in steps similar to those described hereinabove with reference to FIG. 2 for the alloy having one percent of antimony, but at lower temperatures `depending on the cornposition of the remaining liquid. The separation continues until finally a liquid which has a eutectic composition, i.e., about 11.2 percent of antimony by weight, is left behind, which freezes last. Because of the tendency of the screw impeller of the extrusion apparatus to pick up the hard frozen material and leave behind the antimony-rich liquid alloy, the above separation has been found to result in certain ditiiculties, such as uneven cooling and surging of the material, irregular rates of extrusion and even complete stoppages of the extrusion of the material, stalling Vof the equipment during the extrusion, and variations in the composition of the extruded sheath.

The surging of the material has been found to occur because of the tendency of the screw `impeller to churn the liquid antimony-rich portion of the alloy, remaining after the frozen antimony-poor port-ion thereof has been picked up by the screw impeller and has been forwarded toward the die, and then to pick up suddenly the remaining antimony-rich alloy. Such suddenly picked-up portion lis usually the stepwise antimony-enriched, partially frozen antimony-rich portion of the alloy, which in some cases may even have been separated down to the eutectic composition. This lag in being picked up by the screw impeller is explained by a longer interval of time required to cool antimony-rich portions of the lead-antimony alloy. The stoppages of the material and, in many cases, stalling and stoppages of the equipment has been found to occur, primarily, due to the inability of the lead press to forward the suddenly picked-up frozen portion of the original leadantimony alloy, particularly, when the separation of the alloy resulted in a relatively large amount of an alloy portion having eutectic or near eutectic composition, which freezes suddenly at about 485.6 'E to lock the screw impeller so `as to prevent rotation thereof.

ln accordance. with the present inventions, the abovedescribed ditiiculties may be obviated by `a process of heattreating at relatively high temperatures a lead-antimony 4 alloy, containing from about 0.05 percent to about 10.2. percent, preferably from about (Ll percent to about l. 'ipercent of antimony by weight of alloy prior to extrusion According to this process, an alloy having a desired com- L position is heated until molten, and the molten alloy is heated to relatively high temperatures in the range from about l000 F. to about l400 P. The heating is continued until dross, which is formed on Itop of the molten alloy as a thin overlaying crust of lead and antimony oxides, disappear-s and the surface of the molten alloy takes on specular appearance.

The molten alloy may then be used for extrusion immediately after .the dross-melting process by modifying, if necessary, the usual steps of the sheath-extrusion procedure so as to utilize the molten alloy the temperature of which is well above the normal pre-extrusion temperatures. Alternatively, the molten alloy may be cast into ingots and cooled. When needed for extrusion, the ingots are remelted to the usual pre-extrusion temperatures of the process (from about 700 F. to about 800 R), in accordance with the usual continuous sheath-extrusion procedures. In both cases, the extrusion of such heattreated lead-antimony alloy over the cable core proceeds continuously with resultant relatively uniform composition of the extruded sheath.

Without limiting the invention to any theory and merely for the purpose of facilitating the comprehension of the above-identified method, the following mechanism is proposed to explain the change in extrusion properties of the lead-antimiony alloys invquestion. The dross, which consists substantially of lead oxides and antimony oxides, forms, upon melting, vari-ous eutectic compositi-ons with the lead-autimony alloys, `which could be represented by equilibrium diagrams (not shown) completely different rom that for an alloy consisting only of lead and antimony. The molten dross is taken up into the liquid leadantimony solution and is dispersed relatively uniformly throughout the solution. Upon cooling, the various drosscontaining eutectic compositions freeze before the mass of the lead-antimony alloy begins to freeze and form a plurality of nuclei for the framework `to form the spongelike mass of antimony-poor portion of the alloy fil-led with the relatively antimony-rich liquid portion of the alloy. The so-formed sponge-like mass entraps this liquid to such an extent, that upon kneading by the screw impeller the liquid alloy portion is not separated from the relatively solid portion of the alloy but continues to freeze until the whole mass is frozen. Ilt is also believed that `this heat-treatment inhibits, to some extent, a normally dendrltic growth of the metal crystals of the alloy so that a relatively smooth physical appearance of the frozen alloy 1s obtained. As a result of this heat-treatment, the aboveenumerated problems were :substantially eliminated, allowing utilization of presently available continuous-type extrusion apparatus for truly continuous extrusion.

The relationship between the dross-melting temperatures and compositions of relatively pure lead-antimony alloys are presented by a curve i6 of the dross-melting temperature and alloy composite graph shown in FIG. 3. Abscissa of the graph shown in FIG. 3 shows percentage of antlmony by weight of composition in the system,

Ordinate of the same graph gives the minimum drosgI/.

melting temperature for various compositions. For example, an alloy having about one percent of antimony, corresponding to a point 17 on the curve 16, should be heated to at least l095 F. to melt the dross.

Examination of FIG. 3 shows that alloys having relatively low percentages of antimony, within the above- 1nd1cated range of compositions, require to be heated to higher minimum temperatures to melt the dross, and alloys having relatively high percentages of antimony within the same range may be heated to lower minimum temperatures within the above-indicated heating range to melt the dross, the minimum dross-melting temperature increasing substantially inversely proportional to a decreasing antimony content of the alloy.

l. The minimum dross-melting temperatures of the leadantimony alloys may be below or above the melting temjnerature of apure antimony (1l66.7 F.), depending on the lead-antimony alloy composition. Examination of the curve 16 of FIG. 3 shows that lead-antimony alloys containing more than about 0.33 percent of antimony by weight of the composition may be heated below the melting temperature of pure antimony, i.e. below 1166.7 F. However, lead-antimony alloys containing less than about 0.33 percent of antimony by weight of the composition should be heated above 1166.7 F., the minimum drossmelting temperature increasing with decrease in the antimony content of the alloys.

In some cases it may be desirable to prepare an alloy of the type containing from about 0.05 percent to about 10.2 percent of antimony by weight of the alloy to be used subsequently for continuous extrusion. Such an alloy may be prepared by combining a relatively pure lead, such as one of the corroding leads having at least 99.97 percent lead content, with a relatively pure antimony or a master alloy, containing, for example, about 22 percent of antimony by weight of the alloy, in desired proportions.

The combined lead and antimony mixture is then heated in accordance with the hereinabove-described method by heating above the minimum dross-melting temperature for the desired composition. The minimum dross-melting temperature for the desired composition may be obtained by reading the graph shown in FIG. 3. The heated alloy may then be cast into ingots for transportation to a place of use of the alloys in a continuous extrusion or may be used directly in the molten state for the continuous extrusion.

The melting and disappearance of the dross and the resultant specular appearance of the surface of the molten alloy, is only a guide in determining the minimum heattreatment temperatures of lead-antimony alloys being heat treated.

Where high temperatures may be tolerated, to avoid usual observation of the surfaces of the alloy during treatment, the alloy may be heated directly to a temperature higher than that required to produce the prescribed specular surface. Such direct heating may be useful in such cases where for some reasons it is not possible to observe the surface of the alloy, or where the formation of dross is so negligible that no accurate observation might be had. For example, this may occur in cases where the alloy is molten in an enclosed kettle, or in a protective, non-oxidizing atmosphere. In such cases, the alloy may be heated either to a temperature corresponding on the graph to the composition of the alloy, or to at least 1400 F. for high purity corroding leads.

6 It will be understood that the above-described methods are simply illustrative of the application of the principles of the invention. Other methods may be devised by those skilled in the art which Will embody the principles of the invention and fall Within the spirit and scope thereof.

What is claimed is: 1. The method of preparing and using a lead-antimony alloy for continuous extrusion, which comprises:

combining and heating a pure lead having at least 99.97% lead content with antimony to obtain a leadantimony alloy in a molten state consisting of from about .05% to about 10.2% of antimony by Weight of alloy, the balance being lead, heating the molten alloy to such a relatively high temperature in a range of from about 1000 F. to about 1400 F. that dross formed on top of the molten alloy at lower temperatures and including primarily oxides of lead and antimony disappears and the surface of the molten alloy takes on a specular appearance, thereafter reducing the temperature of the molten alloy to a pre-extrusion temperature of from about 700 F. to about 800 F., and continuously extruding the heat-treated lead-antimony alloy over a cable core. 2. The method of preparing and using a lead-antimony alloy for continuous extrusion, which comprises:

combining and heating a pure lead having at least 99.97% lead content with antimony to obtain a leadantimony alloy in a molten state consisting of from about .05% to about 10.2% of antimony by Weight of alloy, the balance being lead, heating the molten alloy to such a relatively high temperature in a range of from about 1000 F. to about 1400 F. that dross formed on top of the molten alloy at lower temperatures and including primarily oxides of lead and antimony disappears and the surface of the molten alloy takes on a specular appearance, casting the heated alloy into ingots, heating the ingots to a pre-extrusion temperature of from about 700 F. to about 800 F., and continuously extruding the heat-treated alloy about a cable core.

Betterton et al Mar. 13, 1934 Bouton et al June 23, 1936 

1. THE METHOD OF PREPARING AND USING A LEAD-ANTIMONY ALLOY FOR CONTINUOUS EXTRUSION, WHICH COMPRISES: COMBINING AND HEATING A PURE LEAD HAVING AT LEAST 99.97% LEAD CONTENT WITH ANTIMONY TO OBTAIN A LEADANTIMONY ALLOY IN A MOLTEN STATE CONSISTING OF FROM ABOUT .05% TO ABOUT 10.2% OF ANTIMONY BY WEIGHT OF ALLOY, THE BALANCE BEING LEAD, HEATING THE MOLTEN ALLOY TO SUCH A RELATIVELY HIGH TEMPERATURE IN A RANGE OF FROM ABOUT 100*F. TO ABOUT 1400*F. THAT DROSS FORMED ON TOP OF THE MOLTEN ALLOY AT LOWER TEMPERATURES AND INCLUDING PRIMARILY OXIDES OF LEAD AND ANTIMONY DISAPPEARS AND THE SURFACE OF THE MOLTEN ALLOY TAKES ON A SPECULAR APPEARANCE. THEREAFTER REDUCING THE TEMPERATURE OF THE MOLTEN ALLOY TO A PRE-EXTRUSION TEMPERATURE OF FROM ABOUT 700*F. TO ABOUT 800*F., AND CONTINUOUSLY EXTRUDING THE HEAT-TREATED LEAD-ANTIMONY ALLOY OVER A CABLE CORE. 